1. Field of the Invention
The present invention relates generally to forming a blow molded container, and more particularly to a method for forming a blow molded container that increases orientation of material at a region of the blow molded container.
2. Related Art
Conventionally, a container may be manufactured through a process known as blow molding. In blow molding, a parison is received at a blow molding apparatus, and the parison is enclosed by a container mold. The blow molding apparatus inflates the parison by forcing gas into the parison which causes the parison to stretch and take the shape of the container mold. Once the parison has taken the shape of the container mold, the blow molding step is complete and the container is removed from the container mold for further processing.
In some applications of container manufacturing, a deep protrusion may be required at a particular section of a container, most often at a base or at a hand grip of the container. Deep protrusions, when located at the base of the container, are sometimes referred to as “push-ups” since the protrusions push up into the interior of the container. However, employing known techniques to manufacture containers with deep protrusions has various problems. One such problem is the orientation of the plastic material around the deep protrusion. Orientation refers to how closely the molecules in a plastic material are packed together. Orientation of plastic molecules occurs as the plastic material stretches, and the greater the material stretch, the higher the orientation. As the orientation of the plastic molecules increases, the molecules straighten and may form a crystalline structure. Typically, the higher the crystallinity of the plastic, the greater the rigidity of the plastic, which improves the structural integrity of the container. The structural integrity of the container may be important during hot fill processing as the container must be able to withstand the rigors of hot fill processing.
In a hot fill process, a product is added to the container at an elevated temperature, about 82° C., which can be near the glass transition temperature of the plastic material, and the container is capped. During hot fill processing and in the subsequent cooling, the container base may experience roll out, distortion, or deformation that can cause the container to lean or become unstable. This problem can be reduced or eliminated by increasing orientation of material in the container base.
During blow molding of a container, gas is forced into a parison which causes the parison to inflate and stretch to take the shape of the container mold. However, the parison cools as it contacts the container mold. Cooling of the parison affects its ability to stretch, and thus its ability to orient. While this may not cause problems for most sections of the container, it particularly affects the orientation of the material formed around a deep protrusion. As the parison contacts the deep protrusion, the parison must flow around the protrusion into a recess. As the parison contacts the protrusion and cools, the parison is less able to flow around the protrusion, which affects the ability of the parison to stretch and to orient plastic material at the recess. Insufficient orientation at a region, such as at a base or at a hand grip, may affect the ability of the region to maintain its shape around the protrusion, the strength of the region, or the ability of the container to stand on a flat surface. Cooling of the parison also is known to create thick amorphous plastic sections around the protrusion, which adds excess plastic material to the container and affects the rigidity around the protrusion. The thick amorphous plastic sections add to the weight of the container, and thus the cost.
A known system for manufacturing a blow molded container is described in U.S. Pat. No. 5,255,889 to Collette et al., which is incorporated herein by reference. In the system described therein, a preform is received and enclosed by a mold chamber, which includes two side movable mold members and a base mold. In the mold chamber, the base mold member has an upper base plug with a protrusion that extends upward toward the center of the mold chamber. During blow molding, gas is forced into the preform to inflate and stretch the preform material into the shape of the mold chamber. As the preform material reaches the protrusion, the material stretches around the protrusion into a recess to form a bearing surface of the container. Once the container is formed, the mold chamber (the two side mold members and the base mold member) opens and releases the molded container. However, the base of the containers generated by this system may have limited crystallinity, a build up of amorphous unoriented material, or other problems in the base similar to those described above due to forcing the preform to stretch around the protrusion into the recess to form the bearing surface of the container.
Likewise, FIG. 1 illustrates a base assembly 100 for forming a container base according to the prior art. The base assembly 100 includes a base pedestal 102, a centering pin 120, and a base plug 104, with the base plug 104 being secured to a top surface of the base pedestal 102. The centering pin 120 may be used to secure and position the base assembly in a blow molding apparatus (not shown). The base plug 104 includes a base mold 106 for forming a container base. The base mold 106 includes a protrusion 108 for forming a deep protrusion in the container base, and a surface 110 for forming a bearing surface of the container base.
During blow molding of a parison into a container, the base mold 106 forms the parison material into a base of the container. As the parison material contacts the base mold 106, the parison material stretches around the protrusion 108 down to the surface 110 for forming the bearing surface, as indicated by the arrows A and B. However, once the parison contacts the protrusion 108, the parison material begins to cool and the orientation of the parison material is slowed, which causes the formation of thick amorphous plastic sections in the base. The thick amorphous plastic sections affect the rigidity of the base, the ability of the container to stand on a flat surface, and add to the cost of the container.
What is needed is an improved system for forming a deep protrusion in a container that overcomes the shortcomings of conventional solutions.